The invention relates to an integrated circuit (IC) that is designed to sense the composition of a multi component automotive fuel that is used to run an engine. As the composition is sensed, the engine operation is modified for the efficient burning of the fuel. Thus, undesirable combustion products are minimized. For example, it has been found desirable to use methanol as well as gasoline to operate automotive engines. Unfortunately, the engine's fuel injection and timing must be changed when shifting from one fuel to another if efficient fuel burning is to be achieved. More desirably, the engine could be automatically adjusted to efficiently burn either gasoline, methanol or mixtures thereof. Then, it doesn't matter what fuel is present in the tank and refueling does not need to take into account what the previous refueling involved. One way to do this is to locate a fuel sensor in the engine fuel feed line and determine what fuel mixture is being sent to the engine and to use this information to program an engine control module (ECM) thereby to optimize engine performance. The sensor is located ahead of the fuel injectors at a spacing that will produce an adequate time to set engine performance as desired by the time the measured fuel reaches the engine. This delay is determined by the fuel flow rate at the optimum vehicle speed.
The fuel sensor must be capable of distinguishing between gasoline and methanol and mixtures thereof. It has been found that a capacitance measuring sensor is useful because the dielectric constants of gasoline and methanol are substantially different. As shown in FIG. 1, which displays a graph of capacitance versus percentage of methanol by volume, a substantial change in capacitance is present. Over most of the mixture range the capacitance variation is linear. For 100% gasoline the capacitance, for the test sensor, is about 28 picofarads. For 100% methanol the capacitance is about 338 pico-farads. The sensor itself consists of a fuel line section that has a small wire element located coaxially in the tubing bore. Ideally, the sensor can be located in the fuel line without measurably changing the dynamic fuel flow characteristic. If desired, a section of the fuel line is insulated from the remainder of the line and both the line and the coaxial wire are coupled to the IC. However, if uninsulated, the fuel line itself is at ground potential so the coaxial wire forms a capacitor plate that is referenced to ground.
The sensor capacitance will yield the methanol percentage whose information will be adequate to set the engine performance by way of the ECM. However, another factor must be taken into account in the form of fuel conductivity. Pure gasoline is substantially nonconductive and the introduction of a contaminant, such as water, will not significantly alter its conductivity. In the case of methanol, such water contamination will significantly alter its conductance. This change in conductance must be dealt with in the capacitance sensing system. As shown in FIG. 2, the methanol-gasoline mixture displays a strong resistance-proportion relationship. Below 30% methanol the resistance is in excess of 15 k ohms and its effect is easily avoided. However, at a 50% volume mixture the shunt resistance is about 7.5 k ohms and decreases substantially as the methanol percentage rises. More importantly as shown in curve 11, water contamination becomes significant. The water content does not significantly change engine performance and its presence does not require compensation. However, as evident in curve 11 of FIG. 2, a small amount of water will substantially increase the conductivity of the mixture. At 30% methanol the presence of water will reduce the sensor resistance from over 15 k ohms to well below 11 k ohms. At 50% methanol the shunt resistance drops from about 7.5 k ohms to about 3.5 k ohms which is about a 50% drop. Thus, it is clear that some means must be employed to avoid the shunt resistance effect if capacitive sensing is to succeed.